The present invention relates to a vehicle steering system having a steering handle for steering at least one wheel, the steering system having an arrangement for applying a torque to the steering handle and a sensor for detecting the steering torque Tdriver applied to the steering handle by the person steering the vehicle, the arrangement generating a return torque Treturn which is a function F(φsteer) of the steering angle φsteer, and the return torque Treturn driving the steering handle to a central position (φsteer=0°).
The return of the vehicle steering system restores a vehicle that has been turned to straight-line travel. Exact automatic centering by the vehicle steering system itself is also desirable when, inter alia, the person steering the vehicle is not touching the steering wheel or is only applying a very small force to it.
When a vehicle steering system performing an active return is being produced, there are, as things stand at the moment with regard to production, always two conflicting aims which arise relating to the generation of the return torque. On the one hand the vehicle steering system must have good centering with the residual angle being only minimal and on the other hand it must transmit a harmonized feel from the steering, particularly when steering takes place through the central position.
To obtain very good centering with the residual angle being only minimal, a prerequisite is a function curve for the restoring force such as is shown by the dashed curve in FIG. 1. In this case, a return torque or force Treturn is generated down to a steering-wheel angle of 0° (zero degrees), which means that the steered wheels are adjusted to the central position even when travelling slowly. However, due to the discontinuity in the path followed by the curve, the feel given by the steering is unpleasant when steering through the center if the person steering the vehicle has his hands on the steering handle or steering wheel.
However, a harmonized feel from the steering when steering through the zero position (central position) can only be obtained with a torque curve f1(φsteer) for the return torque Treturn of the kind which is indicated by the solid curve in FIG. 1. However, exact centering is not obtained with a return torque Treturn of this kind because, due to the decline in the return torque, there is still a residual angle, particularly at low vehicle speeds and/or when there is a high opposing torque due to friction.
The object underlying the invention is therefore to provide a vehicle steering system in which there is a harmonized feel from the steering with, at the same time, exact centering even at low vehicle speeds.